mwl8k: Disable tx_wait completion

Whenever TX ring is drained, priv->tx_wait still points
to a valid completion. Making sure that it points to
NULL before returning from the  mwl8k_tx_wait_empty
check.

Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>